The increasing demands for clean energy have triggered tremendous research interests on electrochemical energy conversion and storage systems with minimum environmental impact. Electrolysis of water into hydrogen and oxygen provides a promising strategy to store electricity generated from renewable energy sources such as solar and wind. Development of efficient, inexpensive water electrolysis systems, combined with hydrogen fuel cells, will provide continuous usage of intermittent renewable energies with minimum environmental impact. One of the key challenges in commercialization of these systems is to develop electrode materials of high efficiency and low cost.
To replace the precious metal based oxygen evolution reaction (OER) catalysts, e.g. IrO2 and RuO2 in commercial water electrolyzers, non-precious metal based catalysts need to meet the strict requirements, including high current densities (j) (≥500 mA cm−2) at low overpotentials (≤300 mV), and prolonged durability. First-row transition metals, such as Ni, Co and Fe, have been an active area of research during the past few years due to their comparable performances in electrochemical energy systems and significantly lowered costs compared with the precious metals, e.g. iridium, ruthenium and platinum. For example, nickel and nickel based composites are known to be active catalyst materials for OER, which require an overpotential around 350˜450 mV to deliver a j of 10 mA cm−2. Interestingly, metallic composites containing two or several of these metals often exhibit significantly enhanced electrochemical performances, and can satisfy specific applications by adjusting the compositions of the composites. For instance, the incorporation of Fe into nickel oxide (NiO) or nickel hydroxide (Ni(OH)2), either as impurities or the components, results in a greatly improved OER catalytic performance. Furthermore, NiFe and NiFeCo composites have shown considerably high catalytic activity towards OER, and NiCo alloy composites are identified as promising electrocatalysts for hydrogen evolution reaction (HER).
However, known research published to date has failed to achieve results using first-row transition metals that are comparable to precious metal based OER catalysts.
A number of techniques for preparing bimetallic composite electrodes, for example NiFe oxygen electrodes have been described. In a first approach for preparing NiFe based oxygen electrodes, NiFe composites are prepared in bulk and are subsequently coated onto desired substrates with the aid of chemical binders which are generally polymeric in nature. These binders are necessary to build up a robust oxygen electrode, since without the binders, the catalysts loaded onto the substrates can be easily peeled off by the bubbles generated. However, the binders are normally electrical insulating, which will not only decrease the contact area between the electrolytes and the active sites but also diminish the electrical conductivity of the NiFe catalyst, thus leading to greatly receded electrocatalytic performances, greatly inferior to precious metal based OER catalysts.
The second approach for preparing such NiFe oxygen electrodes is to electrodeposit NiFe composites directly onto the surface of 2D planar substrates, such as plates of nickel, stainless steel, platinum and copper. This method only requires simple equipment and the deposits can be easily tuned by adjusting the deposition parameters. Furthermore, the electrodeposited catalysts have certain affinity to the supporting substrates, thereby avoiding the usage of chemical binders. However, catalysts deposited on planar structures always have very limited accessible active sites, since only the few outermost layers are available for OER to take place. Furthermore, bubbles generated during OER tend to accumulate in these 2D structures, which results in voltage drops by blocking the active sites on catalysts and impeding the ionic transportation, again providing performance greatly inferior to precious metal based OER catalysts. Eventually, a considerable amount of bubble overpotential (additional potential required to overcome the voltage drop caused by bubbles formation) is required especially under high current densities.
It would be advantageous to provide a catalytic assembly as an alternative to precious metal based catalysts, which uses metallic composites, and which achieve excellent electrocatalytic performances and prolonged durability. It would also be advantageous to provide electrodes comprising these catalytic assemblies, particularly those that are efficient catalysts towards OER and/or HER.